The present invention relates to a data transmission control method suitable for use to avoid network load concentration, transmission delay fluctuations due to congestion and packet loss in a network which provides a best-effort service like the Internet. The invention also pertains to a program for implementing the method and a data transmission unit using the method.
Since the routing algorithm of the Internet usually chooses the shortest route between nodes, Internet communications are conventionally carried out using only the shortest route.
On the other hand, attempts have been made to adapt the Internet to multi-path routing. There is known in the art a router equipped with functions that, for example, in the case of using an OSPF (Open Shortest Path First) scheme as the routing algorithm, supports decentralization of equivalent traffic loads among multiple routes of equal metrics, and that, in the case of using an IGRP (Interior Gateway Routing Protocol) scheme, unequally distributes the network traffic to multiple routes of unequal metrics in proportion to their metrics.
In the Internet now in use that carries out communications over only one route, network fault and traffic congestion are unavoidable, and when data loss or sever delay are caused by such network fault and traffic congestion, congestion control suppresses the traffic load to the failed route, giving rise to a problem that the throughput cannot be obtained as desired.
A conventional data distribution system in which a router determines distribution ratios for multiple routes based on their metric values does not reflect the ever-changing route conditions in the metric values, and hence this system is not the best distribution system taking into account the condition of each route. This system causes a problem that the throughput on all routes decreases under the influence of some routes of deteriorated conditions.
EdgeStream, Inc. proposes a delivery system in which plural delivery servers capable of sending the same contents in synchronization with each other are provided on different routes of a network, and when the traffic load on the contents transmitting route from the one delivery server to a user exceeds a prescribed value, the contents are sent via the other delivery server, thereby providing a guarantee of moving picture quality at a predetermined transmission rate. This method calls for plural servers, and hence raises the cost accordingly. In addition, this method is not free from the possibility that route switching interrupts the image being played back.
In Japanese Patent Application Laid-Open Gazette No. 2000-216817 there is described a system in which each router on the network monitors the traffic state on the transmission line and, when traffic concentration exceeding the premeasured throughput of a route, switches the route to another as a detour. However, this method requires premeasuring the throughput of each route and generating a routing table of every router on the network on the basis of the premeasured throughputs; hence, this method is impractical.